High brightness light emitting device with small size

ABSTRACT

This application describes a light emitting device or an assembly of light emitting devices. In the completed device, an LED at least partially overlies a thin film transistor and a reflective layer is disposed between the LED and the thin film transistor. Methods to fabricate such devices and assemblies of devices are also described.

RELATED APPLICATIONS

This is a continuation application of U.S. patent application Ser. No.16/200,654 filed Nov. 27, 2018, and is related to U.S. patentapplication Ser. No. 15/984,370 filed May 20, 2018, which are assignedto the assignee hereof and filed by the inventor hereof and which isincorporated by reference herein.

FIELD

The embodiments of the invention are directed generally to lightemitting devices with high brightness and small area.

BACKGROUND

Light emitting devices can form displays when integrated in arrays.Light emitting devices often comprise a light emitting diode (LED) and atransistor. There is a large interest in fabricating displays comprisingmany individual LEDs with small area, sometimes called micro LEDs, withminimal spacing between the LEDs, resulting in a large density of LEDs.The current manufacturing steps to fabricate light emitting devices withsmall LEDs and transistors in high density are not cost effective. It isadvantageous to reduce the cost of assembling small LEDs into lightemitting devices. In addition, it is advantageous to increase thebrightness of small light emitting devices.

SUMMARY

Embodiments described herein provide for a light emitting device withsmall size, high brightness, and lower cost manufacturing steps thanconventional methods. In one embodiment, a light emitting devicecomprises a thin film transistor, an LED, wherein the LED at leastpartially overlies the transistor, and a reflective layer disposedbetween the thin film transistor and the LED. The LED has a surfacearea, which is the area in a plane parallel to the LED substrate, whichcan be small, for example less than 40,000 square microns.

Another embodiment is an assembly of light emitting devices comprising aplurality of thin film transistors, a plurality of LEDs wherein each LEDof the plurality at least partially overlies a thin film transistor ofthe plurality of thin film transistors, a reflective layer, wherein thereflective layer is or are disposed between an LED of the plurality ofLEDs and a thin film transistor of the plurality of thin filmtransistors; and a backboard with a plurality of interconnects. The LEDscan have a small area, for example less than 40,000 square microns perLED.

Another embodiment is a method to fabricate a light emitting devicecomprising the steps of forming a structure, the structure formed bysteps comprising: a) forming an LED on an LED substrate; b) forming areflective layer overlying the LED; c) forming a thin film transistoroverlying the LED, wherein a first interconnect extends through thereflective layer electrically connecting the LED to the thin filmtransistor; d) forming a dielectric layer overlying the thin filmtransistor; e) forming a second interconnect and a third interconnectextending through the dielectric layer, wherein the second and thirdinterconnects are electrically connected to the thin film transistor;and flipping the structure comprising the LED, the reflective layer, thethin film transistor, the first, second, and third interconnects, andthe dielectric layer, so that, in the completed device, the LED at leastpartially overlies the thin film transistor, and the reflective layer isdisposed between the thin film transistor and the LED.

Another embodiment is a method to fabricate an assembly of lightemitting devices comprising the steps of: forming a structure, thestructure formed by steps comprising: a) forming a plurality of LEDs onan LED substrate; b) forming a reflective layer; c) forming a pluralityof thin film transistors, wherein each thin film transistor of theplurality of thin film transistors at least partially overlies an LED ofthe plurality of LEDs; d) forming a first plurality of interconnects,wherein each interconnect of the plurality extends through thereflective layer; e) forming a dielectric layer overlying the thin filmtransistors; f) forming a second plurality of interconnects extendingfrom the thin film transistors through the dielectric layer; flippingthe structure comprising the plurality of LEDs, the reflective layer,the plurality of thin film transistors, the first and second pluralityof interconnects, and the dielectric layer, so that, in the completeddevice, each LED of the plurality of LEDs at least partially overlies athin film transistor of the plurality of thin film transistors; andbonding the second plurality of interconnects to a backboard.

BRIEF DESCRIPTION OF THE DRAWINGS

The drawings are not to scale, and the thickness and dimensions of somelayers may be exaggerated for clarity. These and other features,aspects, and advantages of the embodiments described herein will becomebetter understood with regard to the following description, appendedclaims, and accompanying drawings, where:

FIG. 1 illustrates a cross sectional view of one embodiment of a singlelight emitting device wherein the thin film transistor has been formedby a “gate first” method and a transparent conductive layer is connectedto one of the conductive layers of the LED.

FIGS. 2A-2J illustrate cross sectional or isometric views of steps ofone embodiment in the fabrication of a light emitting device.

FIG. 3 illustrates a cross sectional view of one embodiment of a lightemitting device wherein the thin film transistor has been formed by a“gate first” method.

FIG. 4 illustrates a cross sectional view of one embodiment of a lightemitting device wherein the thin film transistor has been made by a“gate last” method, and a transparent conductive layer is connected toone of the conductive layers of the LED.

FIG. 5 illustrates a cross sectional view of one embodiment of a lightemitting device wherein the LED has been made by a “gate last” method.

FIG. 6A illustrates a cross sectional view of one embodiment of a lightemitting device wherein the thin film transistor has a vertical channel.FIG. 6B illustrates an isometric view of the thin film transistor-onlyof FIG. 6A with a vertical channel before light emitting devicefabrication is complete. The thin film transistor shown in FIG. 6A hasbeen “flipped over” when compared to the view of the thin filmtransistor in FIG. 6B.

FIG. 7 illustrates a cross sectional view of one embodiment of a lightemitting device wherein the LED at least partially overlies a thin filmtransistor that has a vertical channel, and a transparent conductivelayer is connected to one of the conductive layers of the LED.

FIG. 8 illustrates a cross sectional view of one embodiment of anassembly of light emitting devices wherein each LED at least partiallyoverlies a thin film transistor that has been formed by a “gate first”method, a transparent conductive layer is connected to one of each ofthe conductive layers of the LEDs, and wavelength-converting layersoverlie at least some of the light emitting devices.

FIGS. 9A-9J illustrate cross sectional views of steps in one embodimentin the fabrication of an assembly of light emitting devices.

FIG. 10 illustrates a cross sectional view of one embodiment of anassembly of light emitting devices wherein the wavelength-convertinglayers are disposed on different planes above the LEDs.

FIG. 11A illustrates a cross sectional view of one embodiment of anassembly of light emitting devices with side reflectors disposed betweenadjacent LEDs of adjacent light emitting devices. FIG. 11B illustrates aplan view of one embodiment of an assembly of light emitting deviceswherein side reflectors surround LEDs of approximately equal area.

FIG. 11C illustrates a plan view of one embodiment of an assembly oflight emitting devices wherein side reflectors surround LEDs of unequalarea.

FIG. 12 illustrates a cross sectional view of an embodiment of anassembly of light emitting devices wherein the thin film transistorshave been formed by a “gate first” method and wavelength-convertinglayers exist on one plane above the LEDs.

FIG. 13 illustrates a cross sectional view of one embodiment of anassembly of light emitting devices wherein the thin film transistorshave been formed by a “gate last” method, side reflectors are disposedbetween adjacent LEDs of adjacent light emitting devices, a transparentconductive layer is connected to one of each of the conductive layers ofthe LEDs, and wavelength-converting layers are disposed on one planeabove the LEDs.

FIG. 14 illustrates a cross sectional view of one embodiment of anassembly of light emitting devices wherein the thin film transistorshave been formed by a “gate last” method and wavelength-convertinglayers are disposed on different planes above the LEDs.

FIG. 15 illustrates a cross sectional view of one embodiment of anassembly of light emitting devices wherein the thin film transistorshave vertical channels and wavelength-converting layers are disposed ondifferent planes above the LEDs.

FIG. 16 illustrates a cross sectional view of an embodiment of anassembly of light emitting devices where the thin film transistors havevertical channels, a transparent conducive layer is in contact with theLEDs, and wavelength-converting layers exist on different planes abovethe LEDs.

DETAILED DESCRIPTION OF THE DISCLOSURE

Some embodiments of a light emitting device described herein comprise anLED, a reflective layer, and a thin film transistor. The thin filmtransistor modulates the current supplied to the LED. Other embodimentswill comprise elements to connect light emitting devices to a backboard.In the embodiments described herein, singulation of LEDs is avoided.Singulation describes the separation of one LED die from a groupfabricated on an LED substrate, such as a semiconductor wafer.Conventional methods of singulation include sawing the LED substrate,which results in kerf loss where the sawing blade traveled through theLED substrate. The kerf can be as much as 100 microns wide. Kerf lossincreases the cost of LEDs due to inefficient use of LED substratespace. Kerf loss during light emitting device separation is minimized inthe embodiments described herein. In conventional methods of lightemitting device fabrication, the singulated LED is then electricallyconnected to a transistor using, for example, mechanical placement ofthe LED and wire bonding. Mechanical placement of single LEDs and wirebonding are costly manufacturing steps. In embodiments of the presentinvention, many devices are fabricated concurrently on a substrate,reducing costs compared to fabrication of single a light emittingdevice. Instead of singulation of one light emitting device, groups oflight emitting devices are diced, which reduces total kerf loss, andallows smaller light emitting devices with smaller pitch to bemanufactured. The pitch is the distance from the edge of one lightemitting device to the same edge of an adjacent light emitting device.The size and pitch of the light emitting devices are reduced compared toconventional devices. The completed light emitting device(s) describedherein can be used as part of a display. However, it should berecognized that embodiments described herein can have a broad range ofapplications.

An embodiment of a single light emitting device will first be described.A detailed description of an embodiment to fabricate a single lightemitting device will follow. Other embodiments of single light emittingdevices will be then described. An embodiment of an assembly of lightemitting devices will be described. A detailed description of anembodiment to fabricate an assembly of light emitting devices willfollow. Other embodiments of assemblies of light emitting devices willbe then described. In the figures shown, the LED and thin filmtransistor are of similar size for clarity, but in general, the LED willbe much larger than the thin film transistor. The area of the LEDdescribes the area of the LED that is substantially planar to the planeof the substrate on which it is fabricated. For example, the LED mayhave an area of 40,000 square microns while the thin film transistor mayhave an area of 50 square microns.

When reference is made herein to a method comprising two or more definedsteps, the defined steps can be carried out in any order orsimultaneously (except where context or specific instruction excludesthat possibility), and the method can include one or more other stepscarried out before any of the defined steps, between two of the definedsteps, or after all the defined steps (except where context excludesthat possibility).

Single Light Emitting Device

One embodiment is shown in cross section in FIG. 1. It is to beunderstood that although one light emitting device is shown, the onedevice may be one among many. The light emitting device comprises an LED20, a thin film transistor 24, and a reflective layer 28 disposedbetween LED 20 and thin film transistor 24. The LED 20 may have a smallsurface area, such as less than 40,000 square microns, or less than1,000 square microns, or less than 100 square microns. Thin filmtransistor 24 modulates the electrical current to LED 20, which controlsthe amount of light produced by the light emitting device. Reflectivelayer 28 reflects light emitted by LED 20 in the direction indicated bythe arrow, directing more light towards the viewer. A first interconnect60 extends through reflective layer 28 and electrically connects LED 20to thin film transistor 24. Second interconnect 68 electrically connectsthin film transistor 24 to other devices (not shown). A thirdinterconnect (not shown) electrically connects to the gate 80 of thinfilm transistor 24 in a plane above or below the plane of the page, andis therefore not visible in FIG. 1. The third interconnect will be shownlater in FIG. 2G. Fourth interconnect 70 electrically connects LED 20 toother devices or to ground (not shown). In this embodiment, LED 20completely overlies thin film transistor 24. In other embodiments, theLED may partially overlie the thin film transistor. With thin filmtransistor 24 completely underneath LED 20, other LEDs can be placedadjacent to LED 20 with minimal spacing between them. To summarize, alight emitting device is described comprising a thin film transistor 24,an LED 20 having a surface area at least partially overlying the thinfilm transistor 24, a reflective layer 28 disposed between the LED 20and thin film transistor 24, and an interconnect 60 extending throughthe reflective layer 28 electrically connecting the LED 20 to the thinfilm transistor 24.

By fabricating reflective layer 28 underneath LED 20, the amount oflight directed towards the viewer is increased. By fabricating thin filmtransistor 24 underneath the LED instead of alongside it, adjacent lightemitting devices can be placed closely together, increasing the densityof LEDs per area.

One embodiment of a fabrication sequence to make the embodiment shown inFIG. 1 will now be described. By using whole-wafer process fabricationand avoiding singulation of LEDs, the LEDs in the present invention canbe smaller and more closely packed together. Use of expensiveconventional single device fabrication techniques such as wire bondingcan be minimized. As shown in cross section in FIG. 2A, LED 20 isfabricated on LED substrate 18. LED 20 comprises first conductive layer30 which is disposed on LED substrate 18, an active layer 34 disposed onfirst conductive layer 30, and a second conductive layer 38 disposed onactive layer 34. It is to be understood that this one LED may representone of many LEDs on LED substrate 18.

LED substrate 18 upon which LED 20 is fabricated can be any suitablesingle crystal semiconductor, including but not limited to sapphire,SiC, GaN, ZnO, and Si. LED substrate 18 should be made of single crystalmaterial which is substantially crystal lattice matched with the firstconductive layer 30. Substantially crystal lattice matched is to meanthe mismatch between the two crystal lattices is less than about 25%.For example, GaN and sapphire are substantially crystal lattice matched,with a lattice mismatch of 16%.

First conductive layer 30 and second conductive layer 38 may be anysuitable semiconductor layer known to those skilled in the art,including but not limited to GaP, AlGaAs, GaAsP, AlGaP, GaInP, InGaN,SiC, AlGaN, AlN, InN, or InP, most preferably GaN. First conductivelayer 30 and second conductive layer 38 may the same or differentmaterials. For example, the first and second conductive layers 30 and 38may be GaN doped to opposite polarity, for example, n-type and p-type.Active layer 34 may be fabricated of any standard semiconductormaterials, for example InGaN, in any formation, for example singlequantum well, multiple quantum wells, or double heterostructure. Asknown to those skilled in the art, in other embodiments, there may beso-called current blocking layers (not shown) above and below activelayer 34 (i.e. active layer 34 may be deposited on a current blockinglayer, and a current blocking layer may be deposited on active layer34). The principles and mechanisms of the conductive and active layersare well known to those skilled in the art, and are thus omitted herein.Any suitable method may be used to deposit LED semiconducting materials,including but not limited to metal organic chemical vapor deposition(MOCVD), hydride vapor phase epitaxy, or molecular beam epitaxy, mostpreferably MOCVD.

Referring to FIG. 2B, a first dielectric layer 48 is deposited on LED 20and LED substrate 18. First dielectric layer 48 may be any suitablematerial, including but not limited to SiO₂, Al₂O₃, TiO₂, SiN, orcombinations thereof, most preferably SiO₂. First dielectric layer 48 issubstantially transparent to light of the wavelength emitted by activelayer 34. Substantially transparent is to mean at least 70% of the lightis transmitted through the layer. First dielectric layer 48 can bedeposited by any suitable means, such as evaporation, sputtering,chemical vapor deposition (CVD), or spin-on techniques, most preferablyby CVD. After deposition, first dielectric layer 48 may be planarized.Planarization is the process by which the top surface 32 of firstdielectric layer 48 is rendered substantially parallel to surface 22 ofLED substrate 18. Planarization may be accomplished by any suitabletechnique, including but not limited to chemical mechanicalplanarization (CMP), wet chemical etch, or plasma etch in corrosive gas,most preferably by CMP. Subsequent steps in fabrication will bedescribed assuming that dielectric layer 48 of FIG. 2 has beenplanarized. It is to be understood that the fabrication could proceedwithout this planarizing step. The thickness of first dielectric layer48, which is to mean the distance between surface 22 and surface 32, canbe between 0.05 and 100 microns, most preferably about 2 microns.

A reflective layer 28 is deposited on surface 32 of first dielectriclayer 48. Reflective layer 28 may be any suitable material thatsubstantially reflects visible light of the wavelength emitted by activelayer 34. Substantially reflective is to mean at least 70% of the lightis reflected by the layer. Suitable materials include but are notlimited to metals like aluminum, gold, or silver, alloys of aluminum,gold, or silver, a composite material such as a polymer mixed with metaloxide particles, or a distributed Bragg reflector (DBR), most preferablya DBR. A DBR includes one or more pairs of dielectric layers (notshown). Each dielectric layer in a pair has a different index ofrefraction. The dielectric layers of the DBR may be deposited by anysuitable method, including but not limited to evaporation, sputtering,CVD, or atomic layer deposition (ALD), which is a type of CVD, mostpreferably by evaporation. The thickness of each dielectric layer in theDBR is designed to be about one quarter of the wavelength of light thatis to be reflected. The wavelength of light varies with the index ofrefraction of each material. For example, for an active layer of an LEDthat emits blue light with a wavelength from 440 to 460 nm, a suitablyreflective DBR would be comprised of one to ten, most preferably five,pairs of aluminum oxide (index of refraction is 1.5 to 1.7) and titaniumoxide (index of refraction is 2.3 to 2.7) layers, where the aluminumoxide layer is 67±7 nm thick and the titanium oxide layer is 49±5 nmthick. A reflective layer 28 that is a DBR may comprise silicon oxide,silicon nitride, aluminum oxide, tantalum oxide, or titanium oxide, orany other suitable dielectric materials. While reflective layer 28 isshown as a continuous layer in FIG. 2B, it is to be understood thatreflective layer 28 may not be continuous in all embodiments, and theremay be multiple reflective layers in other embodiments.

Referring to FIG. 2C, a second dielectric layer 100 is deposited onreflective layer 28. Second dielectric layer 100 may be any suitablematerial, including but not limited to SiO₂, Al₂O₃, TiO₂, SiN, orcombinations thereof, most preferably SiO₂. Second dielectric layer 100is substantially transparent to light of the wavelength emitted by theactive layer 34. Second dielectric layer 100 can be deposited by anysuitable means, such as evaporation, sputtering, CVD, or spin-ontechniques, most preferably by CVD. The thickness of second dielectriclayer 100 may be between 0.05 and 10 microns, most preferably about 0.2microns.

A first interconnect 60 extending through reflective layer 28 is nextfabricated. A masking layer such as photoresist (not shown) is appliedand patterned, and portions of layers 100, 28, and 48 are removed byetching. At least a portion of conductive layer 38 is exposed after thisetch is complete. The etching may be accomplished by any suitableetchant, such as corrosive gases like CHF₃, SF₆, HBr, or wet chemicalacid, such as hydrofluoric acid (HF), nitric acid (HNO₃), or sulfuricacid (H₂SO₄), most preferably by corrosive gas. The masking layer isthen removed. First interconnect 60 is then deposited into the cavitywhere layers 100, 28, and 48 were removed, as shown in FIG. 2C. In oneembodiment, first interconnect 60 is cylinder-shaped in plan view crosssection, with the diameter of the cylinder in contact with conductivelayer 38, although any suitable shape may be used. In one example, thediameter of the cylinder-shaped portion of first interconnect 60 is 0.8microns, although any suitable dimension may be used. First interconnect60 may be made of any suitable material that is conductive, and ispreferably substantially transparent to light of the wavelength emittedby the active layer 34, for example light with dominant wavelength 300to 730 nm. For substantial transparency, first interconnect 60 maycomprise ITO, FTO, AZO, metallic nanoparticles, carbon nanotubes,graphene, or conductive polymers such asPoly(3,4-ethylenedioxythiophene, or PEDOT). Examples of firstinterconnect 60 materials that are not transparent may include but arenot limited to titanium, titanium nitride, tungsten, tantalum, tantalumnitride, and aluminum. First interconnect 60 forms a substantially ohmiccontact to conductive layer 38. In some embodiments, conductive layer 38may have had an ohmic contact layer (not shown) formed on it prior tofabrication of interconnect 60. In this case, the ohmic contact wouldoccur between first interconnect 60 and the ohmic contact layer (notshown). First interconnect 60 may be deposited by any suitabletechnique, including but not limited to evaporation, CVD, sputtering,solution deposition, or spray pyrolysis, most preferably CVD.Interconnect material that is deposited on surface 102 of seconddielectric layer 100 may be removed by any suitable technique, includingbut not limited to CMP, wet chemical etch, plasma etch in corrosive gas,most preferably CMP.

Referring to FIG. 2D, in a different embodiment, reflective layer 28 mayhave been patterned (not shown) and etched prior to deposition of seconddielectric layer 100, forming portions of reflective layer 28. Even whenreflective layer 28 is etched into multiple disconnected pieces, it willbe referred to as reflective layer 28 since it was deposited as a singlelayer. In this embodiment, the pattern and etch created opening L1 inreflective layer 28, through which first interconnect 60 extends. Someportion of second dielectric layer 100 is disposed between the twoportions of reflective layer 28 and first interconnect 60. Thefabrication of openings in reflective layer 28 is needed when reflectivelayer 28 is conductive. By etching openings larger than the diameter offirst interconnect 60 and insuring some dielectric material is disposedbetween a conductive reflective layer 28 and first interconnect 60,electrical connection between reflective layer 28 and first interconnect60 is avoided. This technique is not needed when reflective layer 28comprises a dielectric material, such as a DBR.

Referring to FIG. 2E, thin film transistor 24 is next fabricated. In theembodiment shown in FIG. 2E, thin film transistor 24 is fabricated by a“gate first” process. Any suitable thin film transistor process,including but not limited to “gate last” or vertical channel thin filmtransistor process, may be used. In a “gate first” process, the gate 80is first fabricated. A gate layer is deposited using any suitableconductive material, including but not limited to doped silicon, dopedsilicon germanium, doped germanium, or a metal such as molybdenum,aluminum, titanium, titanium nitride, tantalum, tantalum nitride,chrome, tungsten, or tungsten nitride. A masking layer such asphotoresist (not shown) is applied and patterned, and portions of thegate layer are removed by etching, leaving gate 80. The etching may beaccomplished by any suitable etchant, such as corrosive gases like CHF₃,SF₆, HBr, or wet chemical acid, such as HF, HNO₃, or H₂SO₄, mostpreferably by corrosive gas. The masking layer is then removed.

Next the gate dielectric 84 and channel 88 are fabricated. A gatedielectric layer may be deposited by any suitable method, including butnot limited to spin on, sputtering, evaporation, or CVD techniques. Asthe gate dielectric 84 and channel 88 are necessarily etched from alarger initial gate dielectric layer and channel layer, the referencenumbers for each shall refer to the final etched layers. The gatedielectric layer may be any suitable material, including but not limitedto silicon oxide, silicon nitride, aluminum oxide, hafnium oxide,hafnium silicon oxide, or combinations thereof. The gate dielectriclayer is deposited on the gate 80. The channel layer is then depositedon the gate dielectric layer. The channel layer may be deposited by anysuitable method, including but not limited to sputtering, evaporation,or CVD techniques. The channel layer may be any suitable material,including but not limited to silicon, silicon-germanium, germanium, zincoxide, indium zinc oxide (IZO), indium gallium zinc oxide (IGZO), zincoxide nitride (ZnON), or combinations thereof. The channel layermaterial may be amorphous, polycrystalline, or single crystal. A maskinglayer such as photoresist (not shown) is applied and patterned, andportions of the channel layer and gate dielectric layer are removed byetching. The etching may be accomplished by any suitable etchant, suchas corrosive gases like CHF₃, SF₆, HBr, or wet chemical acid, such asHF, HNO₃, or H₂SO₄, most preferably by corrosive gas. The masking layeris then removed. Gate 80 and gate dielectric 84, as shown in FIG. 2E,are fabricated by this step.

Next, the source 94 and the drain 90 are fabricated. A source and drainlayer may be fabricated of any suitable material, including but notlimited to molybdenum, doped silicon, doped germanium, or doped silicongermanium. The source and drain layer is first deposited on channel 88,surface 92 of dielectric layer 100, and interconnect 60. The source anddrain layer may be deposited by any suitable technique, including butnot limited to sputtering, evaporation, or CVD techniques. A maskinglayer such as photoresist (not shown) is applied and patterned, andportions of the source and drain layer are removed by etching. Theetching may be accomplished by any suitable etchant, such as corrosivegases like CHF₃, SF₆, HBr, or wet chemical acid, such as HF, HNO₃, orH₂SO₄, most preferably by corrosive gas. The masking layer is thenremoved, leaving source 94 and drain 90. Drain 90 is in electricalcontact with first interconnect 60, which, in turn, is in electricalcontact with LED 20. Thin film transistor 24 fabrication is nowsubstantially complete.

Referring to FIG. 2F, a third dielectric layer 54 is deposited over thinfilm transistor 24. Third dielectric layer 54 may be planarized by anysuitable technique, such as CMP or etch-back, most preferably CMP,leaving surface 112 of dielectric layer 54 substantially parallel tosurface 22 of LED substrate 18. Subsequent steps in fabrication will bedescribed assuming that third dielectric layer 54 of FIG. 2 has beenplanarized. It is to be understood that the fabrication could proceedwithout this planarizing step. Next, a masking layer such as photoresist(not shown) is applied and patterned, and portions of third dielectriclayer 54 are removed by etching in specific locations. The etching maybe accomplished by any suitable etchant, such as corrosive gas like CHF₃or wet chemical acid, such as HF, most preferably by corrosive gas. Themasking layer is then removed. The cavities in third dielectric layer 54formed by this etching step are then filled with conductive material toform the second interconnect 68, a third interconnect (not visible), anda fourth interconnect 70. The third interconnect is not visible as itexists in a plane above or below the plane of the page in FIG. 2F, andwill be shown in the following FIG. 2G. Second, third, and fourthinterconnects are preferably substantially transparent to light of thewavelength emitted by LED 20, for example light with dominant wavelength300 to 730 nm. Substantially transparent materials for second, third,and fourth interconnects include but are not limited to ITO, AZO, FTO, aconductive polymer such as PEDOT, metallic nanoparticles, carbonnanotubes, graphene, or conductive polymers. Materials for second,third, and fourth interconnects that are not transparent but are stillsuitable include but are not limited to titanium, titanium nitride,tungsten, tantalum, tantalum nitride, aluminum, or combinations thereof.Second interconnect 68 is electrically connected to thin film transistor24. Fourth interconnect 70 extends through dielectric layers 54, 100,and 48, and reflective layer 28, making contact to LED substrate 18.Second, third, and fourth interconnects may be deposited by any suitabletechnique, including but not limited to evaporation, CVD, sputtering,evaporation, solution deposition, or spray pyrolysis, most preferablyCVD. Interconnect material that is deposited on surface 112 of thirddielectric layer 54 may be removed by any suitable technique, includingbut not limited to CMP, wet chemical etch, and plasma etch in corrosivegas, most preferably by CMP. If LED substrate 18 is viewed as thebottom, and third dielectric layer 54 as the top shown in theorientation in FIG. 2F, LED 20 underlies thin film transistor 24. Inother embodiments, LED 20 may partially underlie thin film transistor24. In this embodiment, the LED, the thin film transistor, thereflective layer, the interconnects, and dielectric layers are referredto as the structure at this point in fabrication.

Referring to FIG. 2G, an isometric view of the thin film transistor-onlyis shown. Dielectric layer 24 is not shown in this figure for clarity.Third interconnect 64, which is was not visible in FIG. 2F, is nowvisible and in electrical contact with gate 80. Interconnect 68 is inelectrical contact with source 90, although the point of contact is notvisible in FIG. 2F. Interconnects 64 and 68 are shown with a squarecross section, but may have a circular or other shape cross section.Other elements of the thin film transistor, gate dielectric 84, channel88, and drain 94, are shown for clarity.

Referring to FIG. 2H, the structure comprising LED substrate 18 with LED20 and thin film transistor 24 is now flipped and bonded to a backboard130. Backboard 130 comprises metal wires 134, which extend perpendicularto the page in FIG. 2H. In one embodiment, this technique is sometimesreferred to as flip-chip bonding, as the devices on each respectivesubstrate face one another. In the orientation shown in FIG. 2G, LED 20now overlies thin film transistor 24. After bonding, interconnects 64,68, and 70 are connected electrically to metal wires 134, as shown inFIG. 2I. Interconnect 64 is not visible in FIG. 2I. Any suitable bondingtechnique may be used, including but not limited to thermocompression,anodic, plasma activated, eutectic, or surface activated, mostpreferably anodic bonding. In one embodiment, backboard 130 may be aprinted circuit board (PCB) comprising FR-4 substrate and copperinterconnects. In a different embodiment, backboard 130 may be aninterposer comprising a silicon or glass substrate with metal wires 134comprising copper or aluminum. An interposer can generally providesmaller feature sizes and pitches of metal interconnects compared with aPCB, and an interposer is preferred for this reason. The interposer maycomprise such aspects as metal wires 134 and/or adhesive metal layers(not shown) defined by conventional masking and lithography, etch andmask removal, micro bump connections, ball or stud bumps, copper paste,through silicon vias (TSVs), or any suitable feature needed to connectto the transistors to other devices to form a display. To summarize, amethod to fabricate a light emitting device is describing comprising thesteps of forming a structure, the structure formed by the stepscomprising forming an LED 20 on an LED substrate 18, forming areflective layer 28 overlying LED 20, forming a thin film transistor 24overlying LED 20 wherein a first interconnect 60 extends throughreflective layer 28 electrically connecting LED 20 to thin filmtransistor 24, forming a dielectric layer 54 overlying the thin filmtransistor 24, forming second interconnect 64 and third interconnect 68extending through dielectric layer 54 wherein the second interconnect 64and third interconnect 68 electrically connect to thin film transistor24; and flipping the structure comprising LED 20, reflective layer 28,thin film transistor 24, first interconnect 60, second interconnect 64,third interconnect 68, and dielectric layer 54, so that LED 20 at leastpartially overlies thin film transistor 24 in the completed device, andthe reflective layer 28 is disposed between the thin film transistor 24and the LED 20.

In another embodiment, adhesive metal (not shown) may be used to jointhe second, third, and fourth interconnects 64, 68, and 70 to metalwires 134. Adhesive metal may be deposited on either interconnects 64,68, and 70, or on metal wires 134, and then patterned, and etched, toform layers on top of either interconnects 64, 68, and 70, or metalwires 134. Interconnects 64, 68, and 70 are electrically connected tometal wires 134 at the point of mutual contact with the adhesive metal.The principles and mechanisms of wafer bonding are well known to thoseskilled in the art, and are thus omitted herein.

After bonding, some portion or all of LED substrate 18 may be removed.In other embodiments, LED substrate 18 may not be removed if LEDsubstrate 18 is substantially transparent to light emitted by the activelayer of the LED. LED substrate 18 may be removed by a laser process. IfLED substrate 18 is sapphire, a KrF Excimer laser with an energy densityof 400 mJ/cm², a wavelength of 248 nm, and a pulse width of 38 ns canirradiate the sapphire surface at an elevated temperature of 60° C. andremove the sapphire substrate from first conductive layer 30. If LEDsubstrate 18 is GaAs, a solution of NH₄OH:35H₂O or a solution of5H₃PO₄:3H₂O₂:3H₂O can be applied to remove the GaAs substrate from firstconductive layer 30. If LED substrate 18 is silicon, a solution of KOH,TMAH, HF+HNO₃, or HF+NH₄F can be applied to remove LED substrate 18.

Referring to FIG. 2I, LED 20 and thin film transistor 24 are bonded tobackboard 130 at least at the point of contact to metal wires 134. AfterLED substrate removal, first conductive layer 30 has a surface 132.Surface 132 may be roughened to enhance light extraction from the LED.For example, if first conductive layer 30 is GaN or AlGaInN, surface 132can be roughened by etching in a solution of KOH. If first conductivelayer 30 is GaP, AlGaP, or AlGaNInP, a solution of HCl and H₃PO₄ canroughen surface 132. If first conductive layer 30 is GaP, a corrosivegas mixture of BCl₃, Cl₂, and Ar can roughen surface 132. In otherembodiments, first conductive layer 30 is not roughened.

Referring to FIG. 2J, the roughened surface of conductive layer 30 isnow referred to as surface 142. Transparent conductive layer 140 isdeposited on surface 142 of conductive layer 30. Transparent conductivelayer 140 makes a substantially ohmic contact to first conductive layer30. Transparent conductive layer 140 may any suitable material,including but not limited to indium tin oxide (ITO), aluminum zinc oxide(AZO), fluorine-doped tin oxide (FTO), metallic nanoparticles, carbonnanotubes, graphene, conductive polymers, or very thin layers ofaluminum, silver, nickel, or a stack of a nickel layer followed by agold layer. Transparent conductive layer 140 may be deposited by anysuitable technique, such as evaporation, CVD, or sputtering, mostpreferably evaporation.

A fourth dielectric layer 144 is deposited on transparent conductivelayer 140. Fourth dielectric layer 144 may be any suitable material,including but not limited to SiO₂, Al₂O₃, TiO₂, SiN, or combinationsthereof, most preferably SiO₂. Fourth dielectric layer 144 issubstantially transparent to light of the wavelength emitted by LED 20.Fourth dielectric layer 144 can be deposited by any suitable means, suchas evaporation, sputtering, CVD, or spin-on techniques, most preferablyby CVD. The thickness of fourth dielectric layer 144 may be between 0.05and 10 microns, most preferably about 0.2 microns. The description aboveis a detailed description of one embodiment of a single light emittingdevice and method of manufacture. Several other embodiments aredescribed below.

A different embodiment of a light emitting device is shown in crosssectional schematic in FIG. 3. In this embodiment, during fabrication ofthe LED 220, first conductive layer 200 is only partially etched. Thelight emitting portion LED 220 is defined by the dimension L2 of activelayer 222 while first conductive layer 200 extends beyond dimension L2.In this embodiment, LED 220 is in immediate electrical contact withfirst interconnect 270 by first conductive layer 200, instead of throughan intervening conductive layer, as in the previous embodiment. Thinfilm transistor 224 is fabricated by a “gate first” process.

A different embodiment of a light emitting device is shown in crosssectional schematic in FIG. 4. Thin film transistor 324 is fabricated bya “gate last” process. In the “gate last” process, the source 308, drain304, and channel 310 are fabricated first, followed by the gatedielectric 314, and finally the gate 318. In this embodiment,transparent conductive layer 344 electrically connects conductive layer330 of LED 320 to interconnect 360.

A different embodiment of a light emitting device is shown in crosssectional schematic in FIG. 5. During fabrication of the LED 420, firstconductive layer 400 is only partially etched. In this embodiment, LED420 is in immediate electrical contact to interconnect 460 by firstconductive layer 400, instead of through an intervening conductivelayer, as in the previous embodiment. In this embodiment, thin filmtransistor 424 is fabricated by a “gate last” process.

Another embodiment of a light emitting device is shown in cross sectionin FIG. 6A. Thin film transistor 524 is a vertical channel thin filmtransistor. Thin film transistor 523 is surrounded by dielectric layer532. In vertical channel thin film transistors charge carriers travelbetween source and drain in a direction that is substantiallyperpendicular to the plane of the substrate on which the thin filmtransistor is fabricated. Referring to FIG. 6A, charge carriers travelfrom source 538 to drain 530 through channel layer 534 in the Ydirection. The gate layer 514 is in contact with the gate dielectriclayer 548. Thin film transistor 524 is electrically connected to LED 520by first interconnect 518. LED 520 is electrically connected tointerconnect 560 through first conductive layer 500. Interconnects 560and 518 extend through reflective layer 528. Another interconnectelectrically connects to source 538 in a plane either above or below theplane of the page, and the connection is therefore not visible in FIG.6A.

Referring to FIG. 6B, an isometric view of only the vertical thin filmtransistor 524 is shown. Dielectric layer 552 is not shown in FIG. 6Bfor clarity. Vertical thin film transistor 524 is shown before thestructure is flipped and bonded, and so the device in FIG. 6B is showninverted for clarity, compared to FIG. 6A. Drain 530, channel 534, andsource 538 are arranged in a vertical stack, and are partially enclosedby gate dielectric 548. Gate 514 overlies gate dielectric 548.Interconnect 564, which was not visible in FIG. 6A, is in electricalcontact with source 538, and interconnect 574 is in electrical contactwith gate 514.

Another embodiment of a light emitting device is shown in FIG. 7. Thinfilm transistor 624 is a vertical channel thin film transistor. Thinfilm transistor 624 is electrically connected to LED 620 by interconnect618. LED 620 is electrically connected to interconnect 660 throughtransparent conductive layer 644. Interconnects 660 and 618 extendthrough reflective layer 628.

It is to be understood that the aforementioned embodiments imply nolimitation upon the light emitting device. Any suitable thin filmtransistor may be combined with an LED, with a reflective layer disposedbetween the LED and the thin film transistor.

Assembly of Light Emitting Devices

A description of one embodiment of an assembly of light emitting devicesand its method of manufacture is now described. An assembly refers to aplurality of light emitting devices. The light emitting devices in anassembly may be grouped. The assembly may comprise one or more groups.Referring to FIG. 8, an embodiment of a group of three light emittingdevices 704, 708, and 710 is shown, although it is to be understoodthere may be many more light emitting devices than shown in the group.Each light emitting device, shown by representative light emittingdevice 708, comprises an LED 718, a thin film transistor 734, and areflective layer 728. Each LED at least partially overlies the thin filmtransistor to which it is electrically connected. In this embodiment,the LEDs are fabricated of the same materials and emit light of the samedominant wavelength. A transparent conductive layer 724 electricallyconnects the LEDs in the group, and a plurality of wavelength-convertinglayers 740 and 744 overlie LEDs 718 and 720, respectively, while LED 714does not have a wavelength-converting layer overlying it. Each of thewavelength-converting layers 740 and 744 is excited by light emitted bythe LED underlying it, and emits light of a different dominantwavelength than the LED. In other embodiments, there may be nowavelength converting layers in the assembly. The devices are bonded tobackboard 810 and the thin film transistor of each light emitting deviceis electrically connected to metal wires 814 after bonding. A group ofthree light emitting devices may form a color pixel in a display, aseach light emitting device may emit light of a different dominantwavelength. To summarize, one embodiment of an assembly of lightemitting devices comprises a plurality of thin film transistors, aplurality of LEDs wherein each LED of the plurality has a surface area,each LED of the plurality at least partially overlies the thin filmtransistor to which it is electrically connected, a reflective layerdisposed between the plurality of LEDs and thin film transistors, and abackboard with a plurality of metal wires.

Referring to FIG. 9A, one embodiment of a fabrication method for anassembly of light emitting devices will now be described. Many of thesteps in fabrication of an assembly are the same as the previousdescription for a single device, and those steps will be summarized.Where the steps are different or new, a more detailed description willbe provided. As shown in FIG. 9A, LEDs 718 are fabricated on LEDsubstrate 701. It is to be understood that these three LEDs can be amongmany more LEDs fabricated on LED substrate 701.

Referring to FIG. 9B, a first dielectric layer 760 is deposited on theLEDs and LED substrate 701. After deposition, first dielectric layer 760may be planarized rendering its' top surface 762 substantially parallelto surface 702 of LED substrate 701. A reflective layer 764 is depositedon first dielectric layer 760. While reflective layer 764 is shown as acontinuous layer in FIG. 9B, it is to be understood that the reflectivelayer 764 may not be continuous in the completed assembly in allembodiments, and there may be multiple portions of the reflective layerin other embodiments.

Referring to FIG. 9C, a second dielectric layer 768 is deposited onreflective layer 764. A first plurality of interconnects 770 extendingthrough reflective layer 768 are next fabricated. First plurality ofinterconnects 770 may be made of any suitable material that isconductive, and is preferably substantially transparent to light of thewavelength emitted by the active layer 754 of the LEDs, for examplelight with a dominant wavelength of 300 to 730 nm. First plurality ofinterconnects 770 form substantially ohmic contacts to conductive layers750. In some embodiments, conductive layers 750 may have had an ohmiccontact layer (not shown) formed on them prior to fabrication ofinterconnects 770. In this case, the ohmic contact layer would bebetween interconnect 770 and the unshown ohmic contact layer, and wouldfacilitate electrical connection between interconnect 770 and conductivelayer 750. Interconnect material that is deposited on surface 772 ofsecond dielectric layer 768 may be removed by any suitable technique.

Referring to FIG. 9D, an alternative embodiment of reflective layer 765is shown in schematic cross section. In this embodiment, afterdeposition, reflective layer 765 is patterned (not shown) and etched toremove portions. Second dielectric layer 768 is then deposited onreflective layer 765 and in the cavities where reflective layer 765 wasremoved. First plurality of interconnects 770 are formed in dielectriclayers 768 and 760, making electrical contact to conductive layer 750.In this embodiment, first plurality of interconnects 770 are insulatedfrom reflective layer 765 by second dielectric layer 768.

Referring to FIG. 9E, thin film transistors are next fabricated. In thisembodiment, the thin film transistors are fabricated by a “gate first”process. However, any suitable thin film transistor process, includingbut not limited to “gate last” or vertical channel thin film transistorprocesses, may be used. Representative thin film transistor 738comprises the gate 774, the gate dielectric 778, the channel 780, thesource 784, and the drain 788.

Referring to FIG. 9F, a third dielectric layer 800 is then depositedover the thin film transistors. The second plurality of interconnects794, including some interconnects that are not visible in FIG. 9F, andthe third plurality of interconnects 798 are then formed. The second andthird plurality of interconnects are preferably substantiallytransparent to light of the wavelength emitted by the active layer ofthe LEDs, for example light with dominant wavelength 300 to 730 nm. Eachinterconnect in the second plurality of interconnects 794 electricallyconnects to a thin film transistor 738. The interconnects in the thirdplurality of interconnects 798 contact LED substrate 701 at this pointin fabrication. An interconnect electrically connects to gate 774 in aplane above or below the plane of the page, and the connection istherefore not visible in FIG. 9F. This unseen interconnect is shown inthe single device in FIG. 2G. In this embodiment, the plurality of LEDs,the reflective layer, the plurality of thin film transistors, theinterconnects, and the dielectric layer are referred to as a structureat this point in fabrication.

Referring to FIG. 9G, the structure is now flipped over in preparationfor bonding to the backboard 810. Backboard 810 comprises metal wires814. The metal wires are shown in cross section in FIG. 9G, and extendperpendicular to the page. Referring to FIG. 9H, backboard 810 is bondedto the thin film transistors and LEDs at metal wires 814. After bonding,interconnects 794 and 798 are connected electrically to metal wires 814.

In another embodiment, adhesive metal (not shown) may be used to jointhe interconnects 794 and 798 to metal wires 814. Adhesive metal may bedeposited on either interconnects 794 and 798, or metal wires 814,patterned and etched, to form layers on top of either interconnects 794and 798, or metal wires 814. Interconnects 794 and 798 are thenelectrically connected to metal wires 814 at the point of mutual contactwith the adhesive metal.

After bonding, some thickness or all of LED substrate 701 may beremoved. In other embodiments, LED substrate 701 may not be removed ifthe LED substrate is substantially transparent to light emitted by theactive layer of the LEDs. In subsequent figures, LED substrate 701 iscompletely removed.

After LED substrate removal, first conductive layer 758 has a surface812 as shown in FIG. 9H. Surface 812 may be roughened to enhance lightextraction from the LED. In other embodiments, first conductive layer758 is not roughened. In the orientation shown in FIG. 9H,representative LED 718 now overlies representative thin film transistor734 after bonding.

Referring to FIG. 9I, after roughening, the surface layer of conductivelayer 758 is referred to as surface layer 902. Next, transparentconductive layer 724 is deposited on surface 902 of conductive layer758. Transparent conductive layer 724 makes a substantially ohmiccontact to first conductive layer 758.

Referring to FIG. 9J, a fourth dielectric layer 910 is deposited ontransparent conductive layer 724. Fourth dielectric layer 910 issubstantially transparent to light of the wavelength emitted by activelayer 754.

Wavelength-converting layer 740 is next deposited on fourth dielectriclayer 910. In embodiments where there is no dielectric layer ontransparent conductive layer 724, a wavelength-converting layer isdeposited directly on transparent conductive layer 724.Wavelength-converting layer 740 will be excited by the light emitted bythe active layer 754 and emit light of a different wavelength. In aspecific embodiment, wavelength-converting layer 740 may be an organicmaterial, for example silicone, epoxy, or polycarbonate, combined withinorganic phosphor particles such as GaAlN, with the GaAlN phosphorparticles excited by light with a dominant wavelength of between 420 and470 nm (blue) and emitting light with a dominant wavelength of between500 and 550 nm (red). In a different embodiment, the phosphor particlesmay be CaAlSiN:Eu, with CaAlSiN:Eu phosphor particles excited by lightwith a dominant wavelength of between 420 and 470 nm (blue), andemitting light with a dominant wavelength of between 610 and 730 nm(green). Any suitable phosphor particle may be used. In anotherembodiment, silicone, epoxy, or polycarbonate may be combined withquantum dots, the quantum dots being nano-scale semiconductor material,for example group II-VI material having a composition ZnCdMgSe, CdSe,InP, ZnS, K₂SiF₆:Mn, SrLi₃Al₃N₄:Eu, or CsPbBr₃. In one embodiment, thecomposition of the quantum dots can be optimized to emit red (500 to 550nm) or green (610 to 730 nm) light when excited by blue light (420 to470 nm). Any suitable quantum dot composition may be used. In oneembodiment, wavelength-converting layer 740 may be screen printed intoany suitable pattern on the wafer.

In a different embodiment, wavelength-converting layer 740 may bephosphor particles or quantum dots embedded in glass, a technique knownas phosphor-in-glass (PiG) or quantum dot-in-glass (QDiG). In oneembodiment, glass paste combining SiO₂, B₂O₃, phosphor particles orquantum dots, adhesive, and organic solvent may be screen printed in apattern on fourth dielectric layer 910 or transparent conductive layer724 using a stencil. In a different embodiment, the glass paste withphosphor particles or quantum dots may be screen printed without apattern. The volatile organic components of the glass paste may beremoved by drying at 150° C. and sintering at 600° C. In a differentembodiment, the PiG or QDiG layer may be deposited by a spin-ontechnique, in which chemicals and phosphor particles or quantum dots aresuspended in organic solvents. The solution is spun as a continuousfilm. The film is dried and annealed, which substantially removes theorganic materials, leaving a glass film with phosphor particles orquantum dots. The glass film with phosphor particles or quantum dots iswavelength-converting layer 740. If wavelength-converting layer 740 wasdeposited as a continuous film, a masking layer (not shown) can beapplied, patterned, and wavelength-converting layer 740 etched.Wavelength-converting layer 740 may be etched by any suitable etchant,including but not limited to acids or corrosive gases. The masking layeris removed. Wavelength-converting layer 740 may be realized in itssmallest form by the technique of depositing PiG or QDiG in continuousfilm form, depositing a masking layer and patterning, etching the film,and then removing the masking layer.

A similar process is repeated to form wavelength-converting layer 744.Wavelength-converting layer 744 may convert light emitted by theunderlying LED to a different wavelength than that emitted bywavelength-converting layer 740. For example, if wavelength-convertinglayer 740 is excited by light with a dominant wavelength of between 420and 470 nm (blue), and emits light with a dominant wavelength of between610 and 730 nm (green), then wavelength-converting layer 744 may beexcited by light with a dominant wavelength of between 420 and 470 nm(blue), and emit light with a dominant wavelength of between 500 to 550nm (red). In this manner, the three LEDs shown in FIG. 9J emit bluelight, but light emitting device 704 will emit blue light, lightemitting device 708 will emit green light, and light emitting device 710will emit red light.

A passivating layer 914 can be deposited over the plurality ofwavelength-converting layers 740 and 744. Passivating layer 914 protectsthe light emitting devices 704, 708, and 710 from moisture intrusionand/or mobile ions which might degrade the device. Passivating layer 914can be any suitable material which is substantially transparent to lightof the wavelength emitted by the active layers of the LEDs and/or thewavelengths emitted by the wavelength-converting layers. In embodimentsin which there are no wavelength-converting layers, dielectric layer 910can act as a passivating layer. If wavelength-converting layers 740 and744 contain organic materials, which are sensitive to elevatedtemperature, passivating layer 914 can be an organic material, includingbut not limited to silicone, epoxy, or polycarbonate. Use of an organicmaterial for passivating layer 914 can avoid high temperature processingthat could compromise wavelength-converting layers 740 and 744 ifwavelength-converting layers 740 and 744 were organic. Ifwavelength-converting layers 740 and 744 are PiG or QDiG, both of whichare less sensitive to elevated temperature than an organic material suchas silicone, epoxy, or polycarbonate, then passivating layer 914 maycomprise either an organic layer such as silicone, epoxy, orpolycarbonate or an inorganic dielectric material such SiO₂, SiN, Al₂O₃,or TiO₂, most preferably an inorganic dielectric material. The inorganicdielectric material for passivating layer 914 can be deposited by anysuitable method, including but not limited to evaporation, sputtering,or CVD, most preferably CVD. If passivating layer 914 is SiO₂, the layermay be between 0.02 and 10 microns thick, in this example about 0.5microns. Fabrication of an assembly of light emitting devices is nowsubstantially complete.

As described, each fabrication step is completed on many devicessimultaneously, reducing the cost of each step per device. As shown inFIG. 9J, with the thin film transistors underneath their respective LED,the space between each LED can be minimized, increasing the density ofLEDs per area. With the size of each LED and the space between LEDsdetermined by photolithographic techniques instead of mechanical sawingthe area of each LED may be very small.

The description above is a detailed description of one embodiment of anassembly of light emitting devices and method of manufacture.Summarizing, in one embodiment, a method to fabricate an assembly oflight emitting devices is described. It comprises the steps of forming astructure, the structure formed by the steps comprising: forming aplurality of LEDs on an LED substrate, forming a reflective layer,forming a plurality of thin film transistors, wherein each thin filmtransistor at least partially overlies an LED of the plurality, forminga first plurality of interconnects wherein each interconnect extendsthrough the reflective layer, forming a dielectric layer overlying thethin film transistors, forming a second plurality of interconnectsextending from the thin film transistors through dielectric layer;flipping the structure comprising the plurality of LEDs, the reflectivelayer, the first and second pluralities of interconnects, and thedielectric layer so that in the completed assembly, each LED of theplurality of LEDs at least partially overlies a thin film transistor ofthe plurality of thin film transistors; and bonding the second pluralityof interconnects to a backboard. Several other embodiments are describedbelow.

FIG. 10 is a schematic cross section of another embodiment of anassembly of light emitting devices. In this embodiment,wavelength-converting layer 1004 is fabricated on fourth dielectriclayer 1010. A fifth dielectric layer 1020 is deposited on top ofwavelength-converting layer 1004. Wavelength-converting layer 1024 isthen fabricated on top of fifth dielectric layer 1020, followed bydeposition of passivating layer 1030 on top of wavelength-convertinglayer 1024.

FIG. 11A is a cross sectional schematic of another embodiment of anassembly of light emitting devices. In this embodiment, a transparentconductive layer 1100 electrically connects one of the conductive layers1110 of each LED 1130. Side reflectors 1150 are disposed around theedges of the LEDs. Side reflectors 1150 prevent light from adjacent LEDsin the plurality of LEDs from interfering with one another. The sidereflectors 1150 also help to directionalize the light, in the directionshown by the arrow in FIG. 11A. Side reflectors 1150 may be any suitablematerial that substantially reflects light of the wavelength emitted bythe LED. Side reflectors 1150 may be any suitable material, includingbut not limited to metals such aluminum, silver, or gold, alloys ofaluminum, silver, or gold, or a composite material such as a polymermixed with metal oxide particles, or a DBR. The metal oxide particlesmay be, for example, titanium oxide. Side reflectors 1150 may bedeposited by any suitable technique, such as sputtering, evaporation,electrodeposition, chemical vapor deposition, liquid dispense, or screenprinting.

FIG. 11B is a plan view of an assembly of light emitting devices shownin FIG. 11A through plane X1-X1. In one example, if side reflectors 1150are conductive, some portion of dielectric layer 1154 will be disposedbetween the side reflector 1150 and LED 1130 to electrically isolateside reflectors 1150 from LEDs 1130. Vertical interconnects 1114 connectto their respective LEDs.

FIG. 11C shows a plan view schematic of a different embodiment of anassembly of light emitting devices, wherein the LEDs do not have thesame area. In this embodiment, LED 1204 has a smaller area than LEDs1208 and 1210. Vertical interconnects 1214 connect to their respectiveLEDs. The LEDs in an assembly may be any suitable shape or size, and theshape and sizes of individual LEDs in one assembly may differ from oneanother. The LEDs may have different areas to account for the varyingbrightness of the light after it has passed through awavelength-converting layers. By fabricating the LEDs with differentareas, the current required to produce a brightness of one color in onelight emitting device may be more closely matched to produce abrightness of light with a different color from a different lightemitting device.

FIG. 12 shows a cross sectional schematic of another embodiment of anassembly of light emitting devices 1300, 1304, and 1308. In thisembodiment, thin film transistors 1340, 1344, and 1348 are fabricated bya “gate first” method, and LEDs 1314 are electrically connected byconductive layer 1311. Wavelength-converting layers 1320 and 1324 aredeposited on the same dielectric layer 1313.

FIG. 13 shows a cross sectional schematic of another embodiment of anassembly of light emitting devices 1400, 1404, and 1408. The thin filmtransistors 1440, 1444, and 1448 are fabricated by a “gate last”process, a transparent conductive film 1411 electrically connects theLEDs 1414, and the wavelength-converting layers 1420 and 1424 aredeposited on dielectric film 1413. Side reflectors 1450 are disposedbetween adjacent LEDs.

FIG. 14 shows a cross sectional schematic of another embodiment of anassembly of light emitting devices 1500, 1504, and 1508. The thin filmtransistors 1540, 1544, and 1548 are fabricated by a “gate last”process. Conductive layer 1511 electrically connects LEDs 1514.Wavelength-converting layer 1532 is deposited on dielectric layer 1513and wavelength-converting layer 1542 is deposited on dielectric layer1530.

FIG. 15 shows a cross sectional schematic of another embodiment of anassembly of light emitting devices 1600, 1604, and 1608. The thin filmtransistors 1640, 1644, and 1648 are fabricated by a vertical channelprocess. Conductive layer 1611 electrically connects LEDs 1614.Wavelength-converting layer 1632 is deposited on dielectric layer 1613and wavelength-converting layer 1642 is deposited on dielectric layer1630.

FIG. 16 shows a cross sectional schematic of another embodiment of anassembly of light emitting devices 1700, 1704, and 1708. The thin filmtransistors 1740, 1744, and 1748 are fabricated by with a verticalchannel process. Transparent conductive layer 1711 electrically connectsLEDs 1714. Wavelength-converting layer 1732 is deposited on dielectriclayer 1713 and wavelength-converting layer 1742 is deposited ondielectric layer 1730.

What is claimed is:
 1. A method to fabricate a light emitting devicecomprising the steps of: forming a structure, the structure formed bythe steps comprising: a) forming an LED on an LED substrate, wherein theLED comprises GaP, AlGaAs, GaAsP, AlGaP, GaInP, InGaN, SiC, AlGaN, AlN,InN, or InP; b) forming a reflective layer overlying the LED; c) forminga thin film transistor overlying the LED, wherein a first interconnectextends through the reflective layer electrically connecting the LED tothe thin film transistor; d) forming a dielectric layer overlying thethin film transistor; and e) forming a second interconnect and a thirdinterconnect extending through the dielectric layer, wherein the secondand third interconnects are electrically connected to the thin filmtransistor; flipping the structure comprising the LED, the reflectivelayer, the thin film transistor, the first, second, and thirdinterconnects, and the dielectric layer, so that, in the completeddevice, the LED at least partially overlies the thin film transistor,and the reflective layer is disposed between the thin film transistorand the LED; and removing the LED substrate after the bonding step. 2.The method of claim 1, further comprising the step of bonding the firstinterconnect and second interconnect to a backboard.
 3. The method ofclaim 2, wherein the backboard comprises a printed circuit board or aninterposer.
 4. The method of claim 1, further comprising forming atransparent conductive layer in electrical contact with the LED.
 5. Themethod of claim 4, wherein the transparent conductive layer comprisesindium tin oxide, fluorine tin oxide, aluminum zinc oxide, metallicnanoparticles, carbon nanotubes, graphene, or conductive polymers. 6.The method of claim 1, wherein the reflective layer comprises a Braggreflector.
 7. A method to fabricate an assembly of light emittingdevices comprising the steps of: forming a structure, the structureformed by the steps comprising: a) forming a plurality of LEDs on an LEDsubstrate, wherein the plurality of LEDs comprises GaP, AlGaAs, GaAsP,AlGaP, GaInP, InGaN, SiC, AlGaN, AlN, InN, or InP; b) forming areflective layer; c) forming a plurality of thin film transistors,wherein each thin film transistor of the plurality of thin filmtransistors at least partially overlies an LED of the plurality of LEDs;d) forming a first plurality of interconnects, wherein each interconnectof the plurality extends through the reflective layer; e) forming adielectric layer overlying the thin film transistors; f) forming asecond plurality of interconnects extending from the thin filmtransistors through the dielectric layer; flipping the structurecomprising the plurality of LEDs, the reflective layer, the plurality ofthin film transistors, the first and second pluralities ofinterconnects, and the dielectric layer, so that, in the completedassembly, each LED of the plurality of LEDs at least partially overliesa thin film transistor of the plurality of thin film transistors;bonding the second plurality of interconnects to a backboard; andremoving the LED substrate after the bonding step.
 8. The method ofclaim 7, further comprising forming a transparent conductive layer onthe plurality of LEDs after removing the LED substrate.
 9. The method ofclaim 8, wherein the transparent conductive layer comprises indium tinoxide, fluorine tin oxide, aluminum zinc oxide, metallic nanoparticles,carbon nanotubes, graphene, or conductive polymers.
 10. The method ofclaim 7, further comprising forming at least first and secondpluralities of wavelength-converting layers, wherein eachwavelength-converting layer overlies an LED of the plurality of LEDs.11. The method of claim 7, further comprising forming a plurality ofside reflectors, wherein each side reflector is disposed betweenadjacent LEDs and the side reflectors prevents light from adjacent LEDsfrom interfering with one another.
 12. The method of claim 7, whereinthe reflective layer comprises a Bragg reflector.
 13. The method ofclaim 7, wherein the backboard comprises a printed circuit board or aninterposer.